Humic substances and napropamide interactions in aqueous solution: A fluorescence spectroscopy study

The complexation of organic solutes with dissolved humic substances (HS) is an important process for the fate of these chemicals in natural aquatic systems. We used the Fluorescence Quenching technique to study the interactions between napropamide, a common herbicide, and ten dissolved humic or fulvic acids extracted from different soils and peats. The primary mechanism of napropamide quenching was determined to be static. The observed complexation constants ranged from 15.9 to 33.8 (L/g. Organic Carbon). Humic acids globally showed a better affinity for complexation with napropamide than fulvic acids with a particular mention for Bouzule and Scheyern humic acids. This may be explained in terms of hydrophobic interactions, in view of the globally hydrophobic nature of napropamide coupled with the fact that humic acids generally contain more hydrophobic regions than fulvic acids. Other possible types of interactions are also suggested. Napropamide binding to HS also depends on the HS elemental composition. It increases with the increase in the (N/C) mass ratio or the HS aromatic fraction but decreases with the increase in O/C mass ratio.